A slide bearing is roughly classified, according to its lubrication type, into a non-lubrication type, a solid lubrication type, and a fluid lubrication type, and the fluid lubrication type is further classified into a water (seawater or fresh water) lubrication type bearing and an oil lubrication type bearing. When the oil lubrication type bearing is used, friction, wear, and seizure can be prevented to some extent by forming a stable oil film on a sliding surface of bearing by a lubricating oil blended with a friction modifier, a viscosity modifier, and the like. On the other hand, the water lubrication type bearing tends to cause severe wear and seizure at a sliding surface due to a direct contact with a counterpart material, since a kinematic viscosity of water serving as a lubricant is much lower than that of lubricating oil. Thus, superior sliding characteristics such as wear resistance are required in the water lubrication type bearing material.
As such, the oil lubrication type bearing was mostly used in the past, however recently there is an increasing need of adopting the water lubrication type bearing in order to prevent environmental pollution in rivers and oceans caused by outflow of the lubricating oil from the bearing. For example, the oil lubrication type bearing is used as a bearing of water turbine, such as a water turbine for hydraulic power generation in the river, pollution of the river caused by outflow of the lubricating oil is becoming a problem. Further, the oil lubrication type bearing is used as a stern tube bearing that supports a propeller shaft in a ship stern tube or a stern tube shaft in a multiple screw ship, pollution of the ocean caused by outflow of the lubricating oil is becoming a problem.
A resin material that is developed for using in a sliding surface of the water lubrication type bearing and thus excellent in water resistance, self-lubricating property, and wear resistance includes a polyurethane resin material to which polyethylene wax is added (Patent Document 1), a thermoplastic resin material such as a polyetheretherketone resin (Patent Document 2), a carbon fiber-containing polyphenylene sulfide (PPS) resin material (Patent Documents 3 to 6), as well as resin materials, such as nitrile-based rubber, ultra high polymer polyethylene (PE), crosslinked PE, polypropylene (PP), and the like (Patent Document 7).
On the other hand, a polytetrafluoroethylene (PTFE) resin is excellent in self-lubricating property, heat resistance, corrosion resistance, as well as processing property. Thus, a carbon fiber (CF)-containing PTFE resin, of which wear resistance is improved by including a carbon fiber (CF), is used as various bearing materials.
However, the PTFE resin material has low wear resistance as compared to the above resin materials. Thus, even if CF is added as a filler, the PTFE resin material is not good enough as a resin material for water lubrication type bearing serving under a severe condition. An embodiment of using a CF-containing PTFE resin has been described (Patent Document 8), however such a resin is used under a special condition where a sliding surface of fixed member as a counterpart material is formed with a titanium nitride (TiN) film.
Further, it has been reported that a resin composition prepared by blending a resin material made primarily of PTFE with a carbon fiber (CF), molybdenum sulfide, a granular inorganic filler having an average particle size of 1 to 50 μm or a whisker having Mohs hardness of 4 or less is used as an impregnation coating composition for a bronze powder layer formed on a porous stainless steel surface of non-circulation type multilayer bearing material (Patent Documents 9 to 11). However, the resin composition requires blending of a solid lubricant such as a granular mineral and the like including molybdenum sulfide and the like, and is used as an impregnation resin composition for the bronze powder layer on the porous surface of the multilayer bearing. Thus, this resin composition is not intended to be singularly used to form a bearing sliding surface.
Therefore, it has been desired to provide a CF-containing PTFE resin composition, which can sufficiently prevent self wear and exhibit excellent sliding characteristics, when used as a sliding bearing material, particularly as a water lubrication type bearing material such as a stern tube bearing material.